A screw machine of this type formed as an extruder is known from DE 1 729 395 B (corresponding to GB 1 277 150). A normal sieve or a filter made of sintered material is provided there as a vacuum housing portion in order to bring about a venting of the bulk material to be processed containing a large amount of air. The porous wall portion may be level and arranged at a clear spacing from the bores in the housing. Alternatively, this porous wall portion may be arranged aligned with the bore walls. It has been shown that, in the latter case, the wall of the porous wall portion facing the bore was clogged by partially plasticized plastics material and therefore the desired effect of venting was destroyed. In the first case, the venting is substantially prevented.
A further extruder is known from DE 195 16 082 A1. There, the basic problem is assumed that feeding powdery bulk material into an extruder is difficult. In order to be able to dispense with the use of pre-compacting loading devices, for example stuffing screws, openings are formed in the intake zone or solid material conveying zone with such a large diameter that gas and solid material can and should discharge from them due to applied negative pressure in the range of 0 to 50%. The single purpose of this negative pressure loading is the degassing of the bulk material. This should furthermore avoid a rearward degassing via the feed opening, which also impedes the bulk material feed. Furthermore, a pressure can already be built up in the intake zone.
A further advantage is to be that an increase in the coefficient of friction on the housing takes place in the region of the openings being used for the discharge of gas and material. It is disadvantageous that apart from gas, part of the powdery solid material is also drawn off through the openings. Thus, adequate operating reliability and economy are not ensured. A similarly constructed feed screw machine is known from DE 20 2007 011 767 U1.
An extruder of the generically assumed type, in which a filter is used as the gas-permeable wall portion, through which air, but also large quantities of process gases, such as, for example steam in the case of ABS and wood flour, are to be drawn off, is known from the publication of “The Japan Steel Works, Ltd., Plastics Processing Technology News Letter”, De-Gassing Cylinder “DGC”. Part of the solid material is also drawn off here through the filter openings. Moreover, the risk also exists of clogging the filter openings by the powdery bulk material.
An extruder similar to the generic screw machine is known from EP 1 977 877 A1 (corresponding to US 2008/0248152 A), in which the gas-permeable wall portion consisting of sintered metal has an inner wall which delimits at least one bore and is set back in relation to the inner wall of the bore with the formation of a gap with a partially annular cross section, wherein, in addition, the gas-permeable wall portion can be attached by means of a pressure-flushing line to a pressure gas source. It has been shown that the gas permeability also decreases here owing to penetrating bulk material particles. In the extreme case, the gas-permeable wall portion becomes unusable owing to clogging of the pores or microchannels, with it also not being possible to eliminate this effect by a back-flush. There is a high outlay in terms of work and cost for exchanging or repairing a wall portion of this type consisting of sintered metal and being gas-permeable per se. One of the reasons for the clogging is that the filter fineness of sintered metals of this type is not small enough, so bulk material particles penetrate into the sintered metal and clog it.